Coating composition

ABSTRACT

The invention discloses a new electroless plating catalyst and a process of using the same for selective plating. The catalyst and process are especially adapted for formation of EMI shielding for electronic components. The electroless plating catalyst comprises particulates dispersed in a liquid film forming composition which particulates are coated with a hydrous oxide that is reduced to a catalytic metal in contact with an electroless plating solution.

BACKGROUND OF THE INVENTION

1. Introduction

This invention relates to a process and materials for electroless metaldeposition. More particularly, this invention relates to a novelelectroless plating catalyst composition comprising a particulate havingan adsorbed coating of a hydrous oxide of a metal, said particulatebeing uniformly dispersed in a liquid coating composition and said metalof the hydrous oxide being a metal that is catalytic to electrolessmetal deposition when contacted with a plating solution containing areducing agent and to a metal coating process using said platingcatalyst. The plating catalyst and process are especially suitable forselective electroless metal deposition such as in the formation ofelectromagnetic interference (EMI) and radio frequency interference(RFI) shielding for housings for electronic equipment.

2. Discussion of Prior Art

Electromagnetic interference emissions are undesirable energy emissionswithin a frequency range of from less than 60 Hz to more than 1,000 MHz.Radio frequency interference (RFI) is the portion of EMI radiation inabout the 0.01 to 1,000 MHz range.

EMI radiation is created by operation of many diverse forms ofelectronic equipment ranging from microwave equipment to home computers.The radiation occurs because electronic devices emit "noise" in theabove frequency range that is picked up by other devices or byconduction through power lines that act as antennas. EMI radiation mayinterfere with other devices and has been known to cause such diverseproblems as interference with police mobile radios, communicationsystems, scientific test equipment and cardiac pacemakers.

In recent years, the use of electronic equipment in the home and workplace has grown rapidly with a concomitant increase in sources of EMIemissions. Additionally, most housings for electronic equipment are nowfabricated from plastic rather than metal. Plastics are lighter, moreversatile, easier to fabricate and less expensive than metal but do notpossess the intrinsic EMI/RFI shielding capabilities provided by metalenclosures.

The Federal Communications Commission has published a series ofregulations concerning standards for maximum allowable EMI emissions forelectronic devices. The regulations, which became effective in October1983, apply to all digital electronic products that use or generatefrequencies between 10 KHz and 1,000 MHz. These regulations thereforeinclude commercial, industrial, business, and home products such ascomputers, calculators, cash registers, electronic typewriters, videoequipment, and electronic games. The regulations require that theelectronics industry develop electronic devices which haveelectromagnetic compatibility (EMC); in other words, equipment whichneither interferes with other devices nor is itself susceptible tointerference.

One approach to limiting electromagnetic containment is the use of anEMI shield to contain the radiation. Containment requires specialshielding materials, components, and structures which prevent generatedenergy from escaping and acting as a source of disturbance.

The effectiveness of electromagnetic containment is determined by thedegree to which the field strength is attenuated as a result ofreflection or absorption by the shielding material. Shielding efficiencyis calculated as a logarithmic function of the ratio of unshielded EMItransmission to shielded EMI transmission and is expressed in decibels.Because of the logarithmic nature, an increase of 30 db in shieldingefficiency for a given wavelength or frequency of electromagneticradiation represents a 1,000 percent increase in the shieldingefficiency of the coating. A coating with a shielding efficiency of 30db, for example, eliminates 99.9% of the total EMI radiation. A 60 dbcoating eliminates 99.9999% of the total EMI radiation.

There are several shielding methods in commercial use for nonconductivematerials. The method most often used involves a metallic coatingapplied over a plastic housing for the electronic device. An earlypublication showing the use of multiple metal coatings over a plastichousing is Engineering, 9, December, 1966, pp. 1,026 and 1,027. Methodsfor applying metallic coatings disclosed in this reference includegalvanic deposition, spray coating, chemical metallizing and vacuummetallizing. Metal coatings include copper, silver, chromium, nickel,silver, gold, zinc, etc.

Metals are applied over housings for electronic equipment in a number ofways. For example, EMI shielding materials have been arc-sprayed (zinc)and painted with metal-containing paints (nickel) onto the electronichousings. Both of these methods have serious disadvantages. Arc-sprayedzinc is an effective EMI shield with attenuation to 120 db or more.However, zinc is toxic and expensive, the procedure is labor intensive,and the coating is prone to cracking and peeling. Conductive nickelpaints are easier to apply than arc-sprayed coatings, but do not coverrecessed areas, provide attenuation to only 20 to 60 db and oftenrequire multiple coatings.

Silver and copper conductive paints have also been used in themanufacture of EMI shielding. Silver is a good conductor, but isexpensive and oxidizes. Copper conductive paints are easy to apply,economical, used with conventional equipment, are wear resistant andhave good resistance to flaking. However, copper tends to oxidize whichresults in a loss of conductivity and a concomitant loss of shieldingeffectiveness.

Other methods for applying metallic coating include cathode sputteringand vacuum metallizing. Such coatings show good conductivity and goodadhesion, but require expensive equipment for application, are prone tomicroscopic cracking, can distort thermoplastics, require high power,are batch operations and are limited by part configuration.

Recently, interest has been generated in the use of electroless metalsfor EMI shielding. Electroless plating of surfaces for EMI shielding isshown in the prior art as early as 1967. Lordi, Plating, Vol. 54, p.382, (1967), incorporated herein by reference, discusses the use of bothelectroless copper and electroless nickel as shielding materials. Lordidiscusses electronic applications for electroless copper and electrolessnickel specifically noting EMI shielding, teaches that electrolessnickel can be used as an intermediate coating over copper to preventcorrosion and finally, that electroless copper can be protected by acoating of a second metal to prevent oxidation.

Recently, a number of publications have discussed the use of electrolessmetals for EMI applications. Plastics Technology, Vol. 27, June '81 p.67, teaches the use of electroless metals as EMI shielding materials.Plastics World, Vol. 40, pp. 40-45, September 1982 states thatelectroless plating may be less expensive than many of the shieldingprocesses now in use and can give comparable shielding performances. Theeconomy of application of electroless plating for EMI shielding isdemonstrated in a 1982 article in Industrial Finishing, Vol. 58, pp. 100to 101. Smoluk, Modern Plastics, September '82, pp. 48-51 cites severalcommercially available electroless plating systems for shieldingapplications. Smoluk reports electroless copper coatings withdemonstrated SE values of 80 to 116 db, and electroless nickel coatingswith SE values exceeding 45 db.

As discussed in the literature, both electroless copper and electrolessnickel have been used in the electroless plating of plastic substrates.Both have advantages and disadvantages. Copper, with a relativeconductivity of 1.0 (second only to silver with a conductivity of 1.05),has high shielding effectiveness. An additional advantage of copper is arelatively low cost. Disadvantages of copper are relatively low abrasion(wear) resistance and a relatively poor corrosion resistance with astrong tendency to oxidize which significantly reduces the shieldingeffectiveness.

Electroless nickel serves as a good paint base, has high wearresistance, stable electrical contact resistance, good solderability,and good corrosion resistance. Plastic Design Forum, November/December1982, pp. 17-26, states that while electroless nickel is less conductivethan electroless copper and therefore less effective as a shieldingmaterial, it possesses better corrosion resistance and may be preferableto electroless copper for EMI shielding applications, especially insevere environmental conditions. The major disadvantage to use ofelectroless nickel is its low relative conductivity of 0.20 or less.However, electroless nickel is relatively expensive and therefore, highcost is a disadvantage to the use of electroless nickel as a shieldingmaterial.

Disadvantages attendant to the use of electroless copper and electrolessnickel separately as shielding materials are partly overcome by a duallayer of electroless copper overplated with electroless nickel. Such adual layer is believed to be first suggested by Lordi (supra) in 1967.In 1983, Krulik, in Industrial Finishing, May, 1983, pp. 16-18, statesthat "the (electroless) copper's disadvantages are overcome by coatingthe copper layer with a thin layer of electroless nickel. Theelectroless nickel is deposited to protect the copper. The nickel'srelative high cost is minimized by the thinness of the layer." A 1983article by Hadju and Krulik in Plating and Surface Finishing, July,1983, pp. 42-44, states that "a composite coating of electroless copperwith a top layer of electroless nickel will combine the desirablecharacteristics of both. There is no degradation of the excellentshielding properties of electroless copper which can be adjusted in itsshielding efficiency by varying its thickness. A relatively thin coatingof electroless nickel provides corrosion resistance, paint adhesion,stable low electrical contact resistance, and other desirable propertiesand may be maintained at a constant thickness". A dual layer ofelectroless copper coated with electroless nickel is also disclosed inU.S Pat. No. 4,514,486 incorporated herein by reference. Thisconfiguration utilizes the high conductivity of the electroless copperfor EMI attenuation and the corrosion resistance of the electrolessnickel to protect the copper against oxidation.

Briefly, electroless plating of plastics comprises immersing a part in aseries of aqueous baths which both prepare the surface of the part fordeposition and permit metallization. Following conventional pretreatmentsteps, a part to be plated is then immersed into a catalyst solutioncontaining noble metals to render nonconductive surfaces catalytic todeposition of the desired plating metal. An example of a noble metalcatalyst is disclosed in U.S. Pat. No. 3,011,920 incorporated herein byreference. The patent teaches treatment of the dielectric substrate witha colloidal palladium solution to render it catalytic to deposition ofthe dissolved metal.

Following catalysis, the part is then immersed into an electrolessplating solution containing dissolved metals which, in contact with theplating catalyst, results in deposition of a coating of the metal ontothe catalyzed surface.

Known procedures for electroless deposition of metal for EMI shieldingare acknowledged by the art to provide superior coatings. However, oneproblem associated with their use is that the coating process is notselective. Coating is by immersion of the entire part to be plated intoa liquid treatment solution--i.e., a colloidal catalyst solutionfollowed by a metal plating solution. The result is that metal is platedover the entire surface of the nonconductor. Where aesthetics areimportant in the marketing of electronic components, a metal coatedhousing for the component is undesirable and typically, the industrypaints the metal coating. This is a time consuming and wasteful step,especially where housings are most often molded in a desired color. Forthis reason, it would be desirable to have a selective process forplating only the interior of the housing without plating the exterior ofthe housing.

An attempt at selective plating of housings for EMI protection isdisclosed in U.S. Pat. No. 4,670,306 incorporated herein by reference.In this patent, a process is taught comprising applying an adsorptivecoating onto selected portions of an electronic housing where plating isdesired. Selectivity is achieved by a masking procedure. This createsareas on the housing of differential adsorptivity. Thereafter, thehousing is immersed in a catalyst solution and more catalyst is absorbedonto the absorptive coating than onto the balance of the housing therebypermitting selective metal deposition. In commercial practice, however,it has been found that selectivity is not adequate because of therequired close control of all plating variables to obtain selectivity.

In published U.K. Patent Application Serial No. 2,169,925 A,incorporated herein by reference, another process for selective platingfor EMI shielding applications is disclosed. In this process, a lacqueris used having suspended particles of metal which may be in the form offlakes, fibers, particulates and in one embodiment, commerciallyavailable silver coated glass spheres. The part to be plated is maskedwhere plating is undesired, spray coated with the lacquer where platingis desired, the mask is removed and the part electrolessly metal platedselectively in a pattern conforming to the lacquer coating. The processof U.K. Application Serial No. 2,169,925 A is an improvement over thatof above referenced U.S. Pat. No. 4,620,306 in that better selectivityis obtainable with fewer processing steps. However, a problemencountered with the process is the need to expose and treat metallic(catalytic) particles embedded in and sealed by the lacquer coatingduring coating and drying to form initiation sites for metallization.The steps of exposing and/or treating the particles prior to plating arecostly and time consuming. Additional problems are encountered due tothe high levels of metal loading in the lacquer which are normally inthe order of 50% w/w. A high loading of metal is costly and the coatingformed using this lacquer is rough in appearance as a consequence of thehigh solids content of the lacquer. Moreover, and possibly due to therough surface, the metallic particles are poorly adhered to thesubstrate, flake off during processing and can fall into circuitrycausing equipment problems and failures.

In copending U.S. patent application Ser. No. 07-531156 filedconcurrently herewith and assigned to the same assignee as the subjectinvention, a new plating catalyst is disclosed that is an improvementover the plating catalyst and process disclosed in the aforesaid U.K.Patent Application No. 2,169,925A. The electroless plating catalystclaimed in the copending application comprises a mixture of severaldifferent catalytic particles dispersed in a liquid coating composition.Each of the catalytic particles comprises a reduced noble metal saltdeposited onto inert carrier particles such as colloidal carbon orsilica. One portion of the catalytic particles comprises a reducedplatinum family metal over a particulate carrier and the other reducedsilver over a particulate carrier. Preferably, the carrier particles areirregularly shaped and possess jagged edges and most preferably comprisecolloidal silica. The liquid coating composition used to carry thecatalytic particles is a resin in a solvent such as a paint or varnishbase and may be organic or aqueous, but is preferably an organic lacquermade using a solvent that softens the top surface of the nonconductorover which metal is to be plated.

In copending U.S. patent application Ser. No. 07-531155 filedconcurrently herewith and assigned to the same assignee as the subjectinvention, another new plating catalyst is disclosed that is based uponthe discovery that the catalytic particles of the platinum family ofmetals may be eliminated from the catalyst without significant sacrificeto the time required to initiate deposition or the quality of thedeposit obtained provided a plating solution with a strong reducingagent is used. Hence, the plating catalyst disclosed in said applicationis similar to that of copending application (attorney docket 38666) inthat it comprises reduced silver over a particulate carrier dispersed ina liquid coating composition but does not contain catalytic particles ofa reduced platinum family metal over a particulate carrier.

SUMMARY OF THE INVENTION

The plating catalyst of this invention is an improvement over theplating catalyst of copending U.S. patent applications Ser. Nos.07-531155 and 07-531156 in that it has now been found that particlesdispersed within a catalytic coating composition may have a hydrousoxide of a catalytic metal adsorbed on their surface which hydrous oxidebecomes catalytic to electroless metal deposition when reduced bycontact with a solution of a reducing agent such as the reducing agentcontained in an electroless plating solution. Accordingly, the platingcatalyst of this invention comprises particles dispersed in a liquidcoating composition, the particles having a hydrous oxide of a catalyticmetal adsorbed on their surface. The hydrous oxide is of a metal that iscatalytic to electroless metal deposition when in reduced form and morepreferably, the hydrous oxide is of a noble metal. The carrier particlesonto which the hydrous oxide is adsorbed are preferably irregularlyshaped and possess jagged edges and most preferably comprise colloidalsilica. The liquid coating composition in which the particles aredispersed to form the plating catalyst is a resin in a solvent such as apaint or varnish base and may be organic or aqueous, but is preferablyan organic lacquer made using a solvent that softens the top surface ofthe nonconductor over which metal is to be plated.

The electroless plating process of the invention is characterized byfewer processing steps and is an improvement over the process of U.K.Patent Application No. 2,169,925A in that it does not require a step ofexposing and/or treating catalytic metal particles prior to electrolessmetal plating as is required in said U.K. Patent Application. Theprocess of the invention permits plating at a good plating rate andresults in a deposit that is and remains strongly adhered to itsunderlying substrate during prolonged use. The invention is especiallyuseful for formation of selectively deposited metal coatings and isespecially suitable for the formation of EMI coatings on housings forelectronic components.

Using a preferred electroless plating catalyst for purposes ofillustration, upon application of the coating composition on a plasticsubstrate, the solvent from the coating composition solvates and softensthe plastic. This results in bonding the polymer phase of the coatingcomposition to the plastic substrate over which the catalyst is coatedwhich in turn bonds the particles within the coating composition to thesubstrate. For reasons to be more fully explained below, the particlesprotrude from the surface of the coating and upon contact with areducing agent such as the reducing agent contained in a electrolessplating solution, the hydrous oxide is reduced to a catalytic form andis readily metal plated without treatment to expose the particles asrequired in the prior art. Additionally, the particles are firmlyadhered to the substrate and do not flake off during prolonged use ofthe article. The metal coating formed over the particles is relativelysmooth compared to coatings obtained using the catalyst of the aforesaidU.K. Patent Application.

The electroless plating catalyst of the invention is especially adaptedfor selective plating. When selectively applied, using conventionalmasking and spraying procedures as more fully explained below,metallization over the substrate will be selective and in conformitywith the applied catalytic coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As aforesaid, the electroless plating catalyst of the inventioncomprises particles dispersed in a liquid coating composition where theparticles have a hydrous oxide of a catalytic metal adsorbed on theirsurface which hydrous oxide becomes catalytic to electroless metaldeposition when reduced by contact with a solution of a reducing agentsuch as the reducing agent contained in an electroless plating solution.The liquid coating composition comprises one or more film forming resinsand other additives dissolved in a solvent.

In accordance with the invention, inert carrier particles over which thehydrous oxide is deposited include known colloidally dispersed particlessuch as carbon; various types of silicas including synthetic calcinedsilicas (produced by heating with a flame), synthetic precipitatedsilicas (produced by chemical reaction), silicas of fossil origin(diatomaceous), detritic natural silicas (powdered or micronized sand);alumina; pigments such as titanium dioxide; etc. Silica is a preferredcarrier. Most preferred is an organically coated silica where theorganic coating aids in the prevention of settling of the silica fromthe coating composition during storage. An example of a preferred silicais ED-30 available from W.R. Grace Ltd.

The particle size of the inert carrier particles may range within widelimits, but preferably ranges between about 0.1 and 500 microns, morepreferably between about 1 and 250 microns and most preferably betweenabout 1 and 50 microns. The surface areas of such particles typicallyrange between 100 and 900 m² /gm.

In a preferred embodiment of the invention, the inert carrier particlesare irregularly shaped rather than smooth such as glass beads.Irregularly shaped particles have jagged edges which penetrate a driedcoating following application and drying of the plating catalyst on asubstrate. This facilitates initiation of the plating reaction without aneed to treat the dried coating to expose the catalytic material.

Various liquid coating compositions containing film forming resins maybe used as the carrier for the catalytic particles of the invention.Useful aqueous based coating compositions include solutions of polymerssuch as acrylic homopolymers or copolymers; polyurethanes; polyamides;polyesters; alkyd resins; ethylene copolymers with acrylates or vinylacetate; chlorinated or unchlorinated homopolymers or copolymers ofvinyl chloride; vinyl acetate or vinyl proprionate; cyclisized orchlorinated rubber; nitrocellulose; ethyl or ethylhydroxy cellulose;coumarine-indene resins; terpene resins; polyvinyl acetal resins;cellulose esters such as cellulose acetobutyrate and celluloseacetoproprionate; shellac; and other natural resins singularly or incombination.

Organic solvent based liquid coating compositions containing filmforming resins include solutions of phenolics; epoxies; polyesters;acrylics; hydroxylated copolymers of vinyl chloride and vinyl acetate;amine or amide resins such as polyamides; urea formaldehyde; melamineformaldehyde resins; hexamethoxymethyl melamine;benzoguanamine-formaldehyde resins; isocyanates; etc.

In general, the particles dispersed in the coating composition areprepared by forming a hydrous oxide of a metal catalytic to electrolessdeposition when in reduced form and adsorbing the hydrous oxide onto thecarrier particles. Metals suitable for formation of hydrous oxides arethose known to be catalytic to electroless metal deposition and includethose more noble than the metal to be plated from solution. Examples ofsuitable metals for formation of a hydrous oxide in accordance with thisinvention include copper, silver, gold, platinum, palladium, indium,rhenium, rhodium, etc. The preferred metals are silver and palladium,silver being more preferred. The least preferred metal in accordancewith the invention is copper because it is poorly active as a catalystand therefore requires a high loading in the coating composition.

The formation of hydrous oxides of many metals, including thosecontemplated herein, is disclosed in U.S. Pat. No. 3,657,003 grantedApril 18, 1972 and in Inorganic Colloid Chemistry by H. B. Weiser, VolII, "The Hydrous Oxides and Hydroxides," Chapter I, John Wiley and Sons,Inc., New York (1935), both incorporated herein by reference. Themeaning of the term hydrous oxide as used herein is in accordance withthe definition in the above cited patent.

Metal hydrous oxides are formed by preparing an aqueous solution of asalt of the desired metal with agitation, pH adjustment and heat asnecessary to obtain complete dissolution of the salt in solution.Thereafter, a hydrolysis and nucleation reaction is permitted or causedto take place at a controlled rate within the solution. This reactiontakes place until a hydrous oxide is formed in situ. The hydrolysisreaction is permitted to continue until the solubility limit of thesolution is exceeded to form a separate, dispersed colloidal phase.Details of the hydrolysis reaction can be found in the above citedreferences and do not constitute a part of this invention though the useof the hydrous oxide colloids to form a plating catalyst is consideredinventive.

Examples of formation of hydrous oxides suitable for use in the subjectinvention follow.

EXAMPLE 1

Formation of a hydrous oxide of palladium--A 1 weight percent solutionof palladium chloride may be dissolved in 100 milliliters of water. Themixture may then be stirred until the maximum amount of palladiumchloride is dissolved. The pH of the solution at this point would beabout 2.7 but would decrease slowly with formation of a murky brownhydrous oxide colloid of palladium.

EXAMPLE 2

Formation of hydrous oxide or palladium--alternative method--10 ml of a5 percent stock solution of palladium chloride may be added to 100 ml ofwater. The initial pH of the solution would then be raised with dilutesodium hydroxide to a pH of about 3.0 to form a brown hydrous oxidecolloid.

EXAMPLE 3

Formation of a hydrous oxide of platinum--A 1 percent solution ofplatinous dichloride may be formed by dissolving the salt in 100 ml ofhot (70° C.) dilute hydrochloric acid. After cooling to roomtemperature, the pH of the solution may then be raised to 3 with sodiumhydroxide to form a yellow hydrous oxide colloid of platinum.

EXAMPLE 4

Formation of a hydrous oxide of copper--A 1 percent solution of cupricchloride may be formed by dissolving the cupric chloride in 100milliliters of water. The solution may then be heated to about 70° C.while the pH would be continuously adjusted as necessary at about 6 withsodium hydroxide. After about 60 minutes of heating, a deep blue hydrousoxide of copper would be formed.

EXAMPLE 5

Formation of a hydrous oxide of silver--a 0.25 weight percent solutionof silver nitrate may be formed by dissolving the salt in 100 ml ofwater with the pH raised rapidly to about 7 with sodium hydroxide. ThepH would then be slowly raised with sodium hydroxide to a range of from8 to 9 to form a milky white hydrous oxide of silver.

EXAMPLE 6

Formation of a hydrous oxide of gold--a 1 percent solution of auricchloride would be dissolved in 100 milliliters of water to produce ayellow solution. The pH would then be slowly raised over a period of 2days to about 4 to 5 with sodium hydroxide. During the raising of thepH, the solution would be continuously stirred and slightly heated toabout 40° C. to form a brown hydrous oxide of gold colloid.

The hydrous oxide, as formed above, may be formed in the presence of adispersed particulate carrier to cause adsorption of the hydrous oxidecolloid onto the particulate carrier as the hydrous oxide is formed.Alternatively, the hydrous oxide may be formed and the particulatecarrier added to the colloidal solution of the hydrous oxide to adsorbthe hydrous oxide onto the carrier. If the plating catalyst is to be anaqueous based composition the particulate carrier having the adsorbedhydrous oxide on its surface may then be dispersed in the coatingcomposition. If the plating catalyst is to be an organic coatingcomposition, the particulate carrier having the adsorbed hydrous oxideon its surface may be recovered by filtration, washed and then dispersedin the coating composition. Dispersion of the particles in the coatingcomposition may be assisted by stirring or preferably ultrasonicagitation to break colloidal agglomerates as is known in the art.

In that embodiment of the invention where the particulate carrier coatedwith the hydrous oxide is dispersed in an organic medium, followingwashing, the particulate carrier may be suspended in an organic solventand the suspension added to the coating composition. The solvent usedtypically is an organic solvent compatible with the liquid coatingcomposition. It is also desirable that the organic solvent solvate orcondition the substrate over which the catalytic coating composition iscoated in order to promote bonding or adhesion of the coating of thecatalyst to the substrate. Solvation of the substrate, as is known inthe art, means softening the substrate without dissolving the same. Thispermits penetration of the substrate by the coating medium.

Solvating solvents for ABS are shown in U.S. Pat. No. 3,445,350incorporated herein by reference. Solvating solvents for other polymersare disclosed in U.S. Pat. No. 3,754,070, also incorporated herein byreference. To solvate the substrate, it is preferred that a polarsolvent be used, more preferably an oxygenated solvent such as analcohol, ether or ether acetate. Suitable solvents include isopropylalcohol, ethanol, methanol, acetone, methyl ethyl ketone, ethyl acetate,the Cellosolve acetates such as butyl cellosolve acetate and propyleneglycol alkyl ether acetate, butyl carbitol, etc. The combination of aspecific solvent with a specific substrate would depend upon thesolvency of the solvent for the substrate. Solvents used to formulatethe electroless plating catalyst can also be used to dilute the same asnecessary for use. A solvating solvent for the substrate may also beadded to the plating catalyst when diluting the catalyst for use.

The concentration of components in the plating catalyst followingdilution with solvent is not critical. The metal content of thecomposition as adsorbed on the particulate carrier (expressed as metal)is preferably in an amount of from about 0.5 to 50.0 grams per liter ofsolution, more preferably, in an amount of from about 1.0 to 25.0 gramsper liter and most preferably, in a range of 1.5 to 10.0 grams perliter. The particulate carrier over which the hydrous oxide is depositedis present in solution in an amount of from 1 to 50 times the weight ofthe metal portion of the hydrous oxide and more preferably in an amountof from 2 to 15 times the weight of the metal.

A made up plating catalyst ready for use will contain the carrierparticles coated with the hydrous oxide, other dissolved solidsconventionally found within a coating composition--i.e., resins,polymers, pigments, etc., all hereinafter collectively referred to asthe "dried coating solids", and the solvent for the coating composition.In a preferred embodiment of the invention, the concentration of thedried coating solids by weight to the suspended particles in the made upplating catalyst varies from about 2 to 1 to 50 to 1 and morepreferably, varies from about 1 to 1 to 25 to 1.

Nonconductive substrates capable of metal plating using the platingcatalyst of the invention include polymers such as polyphenylene oxide,acrylonitrile-butadiene-styrene (ABS) copolymers, polystyrene,polycarbonate, epoxy resins, polyvinyl chloride, polyethylene,polypropylene, polyethylene oxide terephthalate, fluorine polymers suchas polytetrafluoroethylene, and other natural and synthetic polymers andblends of the aforesaid. The substrate may also include non-metallicmaterials such as silicate and non-silicate glasses, for example,quartz, soda lime float or plate glass, borosilicate, lead borate,alumino-silicate, alumina ceramic and tin oxide.

Application of the plating catalyst is by use of standard methods andequipment. A substrate is pretreated using standard methods and thecoating is preferably applied by spraying, although brushing or othermean of selectively applying the coating are applicable. Surfaces thatare not to be coated are masked prior to application of the coating.

Following application of a coating of plating catalyst, the mask isremoved whereby the coating is over the substrate in a selectivepattern. Preferably, the coating is dried prior to electroless plating.Using the coating of the present invention, a recommended period of airdrying is from 15 to 20 minutes followed by oven drying at a temperaturenot exceeding 100° C. for a period of time less than 60 minutes. Thiscures the coating and promotes adhesion of the coating to the substrateand an electroless metal plate over the coating. During drying, solventspresent in the coating of the plating catalyst act to solvate thesubstrate and create a stronger bond between the substrate and thecoating. Entrapped particles microscopically protrude from the surfaceand provide activation sites for subsequent electroless metal depositionfollowing contact with a reducing agent to reduce the hydrous oxide to areduced catalytic form.

Electroless metal plating is accomplished using standard prior artplating solutions such as those disclosed in U.S. Pat. Nos. 3,765,936;3,728,137; and 3,661,597, all incorporated herein by reference. Otherelectroless plating solutions known to those skilled in the art wouldalso be suitable for purposes of the subject invention. As is known inthe art, an electroless plating solution contains a reducing agent whichis necessary to cause the plating reaction to occur. For example, anelectroless copper plating solution contains formaldehyde or a borane oran amine borane. An electroless nickel solution contains hypophosphiteas a reducing agent. When the reducing agent in the electroless platingsolution comes into contact with the hydrous oxide on the surface of theparticles protruding from the coating, it is believed that the contactcauses reduction of the hydrous oxide to a reduced catalytic formwhereby the particles are catalytic and suitable for use as anelectroless plating catalyst. Though this embodiment of the inventionhas been described using a reducing agent within a plating solution toreduce the hydrous oxide, it should be understood that the hydrous oxidemay be reduced by contact with a separate solution of a reducing agentfollowed by contact with the electroless plating solution.

For EMI shielding, a dual layer of electroless copper followed byelectroless nickel is preferred. Such a dual layer is disclosed in theabove referenced U.S. Pat. No. 4,514,486. Using the process of thisinvention, including masking means to obtain selectivity, EMI shieldsare obtained in a selective pattern whereby the aesthetic features of amolded housing, for example, are preserved, while an effective EMIshield is provided.

The invention will be better understood by reference to the exampleswhich follow. These examples are not intended to limit the scope of theinvention.

EXAMPLE 7

This example represents the preferred embodiment of the invention.

Particles having an adsorbed hydrous oxide of silver are prepared usingthe following formulation:

    ______________________________________                                        Silver Hydrous oxide.sup.(1)                                                                          4.00 gm                                               Isopropyl Alcohol       35.00 gm                                              Acetone                 23.00 gm                                              Ethyl Acetate           40.00 gm                                              Silica Type OK 412      19.00 gm                                              Varnish (LS123).sup.(2)                                                                              346.00 ml                                              ______________________________________                                         .sup.(1) Such as prepared in Example 5 and removed by filtration and          washing.                                                                      .sup.(2) L123 Polycarbonate Varnish from Bee Chemicals.                  

The silver hydrous oxide and silica particles are added to the solventswith stirring. Varnish is then added with stirring and ultrasonicagitation to disperse any agglomerates found.

EXAMPLE 8

A portion of a polycarbonate housing was masked and prepared forplating. The formulation of Example 7 was diluted with 50 ml of acomposite thinner (54 volume percent acetone, 22 percent isopropylalcohol and 24 percent ethyl acetate) and sprayed onto the polycarbonatesubstrate. The coating was applied at a wet thickness of about 3 mils at25 psi air pressure. Following spraying, the coated part was allowed toair dry for about 1 hour and then placed in a hot air convection ovenand dried at 100° C. for 10 minutes to provide a final dried coating ofabout 0.5 mil thickness.

Following drying, the mask was removed and the substrate plated byimmersion in a proprietary electroless copper plating solutionidentified as Cuposit® 251 electroless copper and comprising cupricions, complexing agent, formaldehyde and hydroxide for pH control. Theplating solution was maintained at 40° C. Plating was initiated in about2.5 minutes and plating continued for 30 minutes. Following plating, thesubstrate had a coating of copper about 0.1 mil thick deposited only onthe areas of the substrate spray coated with the electroless platingcatalyst. The deposit obtained had a relatively smooth matt pink finishof copper. Testing of suitability of the part as an EMI shield showedresults of <0.02 ohms/sq.

We claim:
 1. A coating composition suitable for use as an electrolessplating catalyst, said composition comprising a polymer solution havingparticles homogeneously dispersed therein said particles comprising aninert particulate carrier coated with a hydrous oxide of a metalinsoluble in the polymer polution that is catalytic to electroless metaldeposition when in a reduced form, said hydrous oxide having a metalcontent of from 0.5 to 50 grams per liter of coating composition.
 2. Thecomposition of claim 1 where the particles comprise a noble metalhydrous oxide adsorbed onto an inert particulate carrier.
 3. Thecomposition of claim 1 where the noble metal is silver.
 4. Thecomposition of claim 1 where the noble metal is palladium.
 5. Thecomposition of claim 1 where the composition is a varnish.
 6. Thecomposition of claim 1 where the polymer is selected from the groupconsisting of phenolics, epoxies, alkyds, polyesters, acrylics,polyurethanes and polyamides.
 7. The composition of claim 1 where themetal concentration varies from about 1.0 to 25.0 grams per liter ofsolution.
 8. The composition of claim 6 where the concentration variesfrom about 1.5 to 10.0 grams per liter of solution.
 9. The compositionof claim 1 where the weight ratio of the inert particulate carrier tothe metal portion of the hydrous oxide varies from 1 to 1 to 50 to 1.10. The composition of claim 9 where the ratio varies from 5 to 1 to 15to
 1. 11. The composition of claim 1 where the inert particulate carrieris chosen from the group of irregularly shaped particles selected fromthe group of carbon, glass, titania or silica.
 12. The composition ofclaim 11 where the particulate carrier is silica.
 13. The composition ofclaim 1 where the particle size of the inert particulate carrier rangesbetween 1 and 100 microns.
 14. A coating composition suitable for use asan electroless plating catalyst, said composition comprising a polymersolution having particles homogeneously dispersed therein, saidparticles comprising a silver hydrous oxide insoluble in said polymersolution coated on an inert particulate carrier, said coatingcomposition having silver present in a concentration of from 0.5 to 50grams per liter of coating composition.
 15. The composition of claim 14where the concentration of silver varies from about 1.0 to 25.0 gramsper liter of coating composition.
 16. The composition of claim 14 wherethe polymer is selected from the group consisting of phenolics, epoxies,alkyds, polyesters, acrylics, polyurethanes and polyamides.
 17. Thecomposition of claim 14 where the weight ratio of the inert particulatecarrier to the metallic silver portion of the silver hydrous oxidevaries between from 1 to 1 to 50 to
 1. 18. The composition of claim 17where the ratio varies between 5 to 1 to 15 to
 1. 19. The composition ofclaim 14 where the inert particulate carrier is chosen from the group ofirregularly shaped particles selected from the group of carbon, glass,titania or silica.
 20. The composition of claim 19 where the inertparticulate carrier is silica.
 21. The composition of claim 14 where theparticle size of the inert particulate carrier ranges between 1 and 100microns.